Health and Safety Services
Safety Note 52
The use of electrical extension systems
Introduction
The number of electrical items of equipment in use at work and at home has proliferated in
recent years. However, the number of power points where we can plug in has not kept pace;
hence the common use of the electrical extension. Extensions come in many forms; the one
common denominator is that they tend to be misused and abused. This Safety Note provides
advice on safe use, to avoid the potential risks of fire or electric shock.
Types of extensions
There are a wide variety of types of electrical extensions, either to increase the distance or to
increase the number of items we can plug in. They include multiblock adaptors, cable
extensions with a single plug point, multi gang bar extension leads and cable reels. Some types
do both tasks and they all have good points as well as bad.
Common Problems
Overloading
The biggest problem with extension leads is overloading. This can be caused by plugging too
many items into the extension bar or plugging one extension lead into another. You should
never lose sight of the fact that they ultimately plug into a wall socket that is rated at a
maximum of 13 amps or 3kW.
Using all the available sockets and overloading the system can cause the extension to overheat;
this is particularly prevalent in systems with coiled cables. There is a chance that in fault
conditions and with added resistance due to multiple fuses and heavy duty cables in the system,
the fuse will not activate until a fire is well alight.
Damage to cables
The other issue is trapping or damage to the cable as it goes through doors and windows, or
when it is walked on. This can lead to the internal wires becoming exposed and damaged, with
the risk of electrical shock.
Exposure to weather/moisture
Any extension that goes outside the building must have a protection device such as a Residual
Current Device (RCD) fitted to the point where it enters the mains power system. The RCD is a
device that constantly compares current in and out. When it detects a change i.e. when the
cable is cut or the current finds a better path to earth via the user, it will trip out and cut the
supply fast enough to prevent death in normally healthy humans. To reduce the likelihood of
injury the RCD must have a tripping current of not more than 30 milliamps (mA). RCDs with a
higher tripping current are used to protect against fire. RCDs must be regularly tested by
pressing the ‘test’ button, and by making sure that the RCD operates. Faulty RCDs must be
removed from use.
The preferred system is to have a dedicated weatherproof RCD supply. If one is not available
and a power supply is frequently needed (such as for regular end-of-term social events,
maintenance etc.) an exterior mounted supply point should be installed. An acceptable
alternative is to use equipment that operates at a reduced voltage such as that supplied by a
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March 2009
transformer with an output that is centre tapped to earth (this halves the voltage between a live
wire and earth). For very infrequent and one-off use out of doors, as a minimum users MUST use
a portable RCD device. This is only acceptable as a temporary measure.
Type
Good points
Multiblock adaptors
Common problems
‘Multiblock’ adaptors can fail or become
loose in the socket, causing overheating.
The use of ‘multiblock’ adaptors is not
permitted in UoR. They should be
removed if discovered.
Single extension leads
Multi gang bar extension
leads
Used to get power to a part of
the room where there is no
existing outlet.
Very simple system but the weak point is
the cable. It is susceptible to damage and
nd
is a trip hazard. Never plug a 2
extension or a 3 way multiblock adaptor
into the end. This would increase the
resistance and the possibility of
overheating, before the fuse blows.
Often used to extend the
outlet point and increase the
number of plugs that can be
connected. They are often
fused and are readily available
at low cost.
They permit too many items to be
plugged in and the single wall socket may
be overloaded.
When the fuse blows, the user may
replace it with a larger capacity one,
thereby increasing the load on the
internal connection points.
As above the cable is a weak point for
damage.
Reel cable extension leads
Built-in RCD cable extension
leads
Used to provide power at
longer ranges. They are very
useful for providing power in
rooms where there is no power
i.e. lofts.
They are often not fully unreeled during
use – this can cause the cable to
overheat.
These are often cable reel
extensions designed for
outdoor use. An excellent
choice for outdoor work - the
RCD device provides a measure
of safety in the event of
damage or water ingress.
There are some types that have the RCD
at the reel end instead of at the plug end.
The reel end types do not provide
protection if the cable of the extension
reel is damaged. The preferred type is the
one with the plug RCD.
They are often taken outdoors with the
added problems of water and risk of
damage.
Health and Safety Services
Governance Directorate
March 2009
SN52, Health and Safety Services
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March 2009